Attempts will be made to transform normal human cells toward malignancy using treatments that are shown to be highly mutagenic for the cells. Transformants will be identified by focus formation, anchorage-independent growth and tumorigenicity in nude mice. Cells from humans with family histories of high cancer incidence will be used. Attempts to identify genetic determinants responsible for transformed traits of human cells will be made with the use of cell fusion, microcell fusion and DNA transfer into suitable recipient cells. Methodologically similar attempts will be made to identify sensescence-initiating mutations in human cells. Evidence for chemical differences between the active and inactive X-chromosomes of mammalian cells will be sought by analysis of restriction enzyme digests of cells having different x-chromosomes constitutions and by means of transfer of inactive X DNA into recipient cells in which derepression of previously inactive genes can be detected. We will attempt to assign sequences of human DNA present in chimaeric DNA molecules to specific human chromosomes using a suitable panel of somatic cell hybrids, emphasizing the identification of X-chromosomal sequences.